1. Field of the Invention
The present invention relates generally to grid-connected power systems and, more particularly, to grid-connected photovoltaic (PV) systems and other grid-connected power systems having a battery or other back-up power source for powering a selected portion of a load when grid power to the load is unavailable.
2. Discussion of the Related Art
Photovoltaic (PV) systems in which PV cells generate electricity from sunlight to power a load have been increasing in popularity due to the ever improving efficiency and cost-effectiveness of PV systems. Stand-alone PV systems are not connected to the electric utility grid and generally comprise an array of PV cells and a battery that is charged with electric current produced by the PV array. Electric current from the battery is used to power the load, with the battery providing a stable power source for the load. Since direct current (DC) electricity produced by the PV array is supplied to a DC battery, current output from the battery will be in a form to power a DC load. Where the load to be powered by the battery is an alternating current (AC) load, current output from the battery is typically supplied to a DC/AC inverter for converting the direct current of the battery into alternating current to be supplied to the load.
Grid-connected power systems have been implemented which are connected to the AC electric utility grid, and one type of grid-connected power system is a grid-connected PV system. In grid-connected PV systems, direct current generated by the PV array is supplied to the utility grid as alternating current via a DC/AC inverter. The utility grid is connected to a load so that power from the utility grid may be used to power the load. A battery is not necessary in grid-connected PV systems since current from the PV array may be supplied to the DC/AC inverter without an intervening battery. However, the DC/AC inverter used in grid-connected PV systems must be capable of converting the direct current of the PV array into alternating current that is compatible with the alternating current of the utility grid. Grid-connected PV systems may be installed at various locations having loads powered by the utility grid. Representative installations include residential and commercial buildings having AC loads such as lighting and appliances.
The power supplied to the utility grid from grid-connected PV systems allows consumers of utility power to reduce their utility power consumption while utilizing a safe, non-polluting, renewable power source. By reducing their utility power consumption, consumers may realize savings in utility costs, particularly where the utility employs a stepped rate schedule by which the rate charged to consumers for utility power increases in accordance with increasing utility power usage. Aside from the economic or commercial benefits, grid-connected PV systems appeal to many consumers for their lack of detrimental environmental impact.
A drawback of many grid-connected PV systems arises when utility service interruptions result in the unavailability of grid power to the load. When this occurs, the DC/AC inverter is required for safety reasons to be disconnected from the utility grid such that the PV system is also disconnected from the utility grid. In the absence of a back-up power source, no power is available from either the utility grid or the PV system to power the load. Consequently, the load may be down until utility service is restored to the load whereupon the DC/AC inverter is reconnected to the utility grid.
In order to address the problem of utility service interruptions, grid-connected PV systems are sometimes provided with a battery forming a back-up power source for powering a selected load while the PV system is disconnected from the utility grid due to unavailability of grid power to the load. Typically, the selected load is less than the entire load normally receiving power from the utility grid and may comprise a selected portion of the load of particular importance to the consumer. The back-up battery may be charged with current from the PV array. Direct current output from the back-up battery is supplied to the DC/AC inverter, which can be electrically connected with the selected load via a transfer switch. Alternating current output from the inverter can be used to power the selected load in a stand-alone mode, such that the inconvenience to consumers resulting from utility service interruptions is minimized.
Prior grid-connected PV systems having a battery back-up utilize back-up batteries providing 48VDC nominal, or less, which disadvantageously limits voltage output from the PV array and voltage input to the DC/AC inverter to 48VDC nominal, or less. At this voltage, the peak conversion efficiency of the DC/AC inverter is usually less than 90% on account of undesirable power losses. In addition, the DC/AC inverter may be undesirably heavy and/or large in physical size and is usually high in cost despite its relatively low voltage input and high inefficiency. DC/AC inverters are available which are capable of stepping up or increasing voltage through the inverters. However, such inverters ordinarily rely on transformers within the inverters, which result in greater efficiency losses through the inverters.
Another disadvantage of prior grid-connected PV systems with a battery back-up relates to the manner in which the battery is charged by the PV array. A grid-connected PV system of Advanced Energy Inc. includes a DC/DC converter receiving as input all of the current coming from the PV array, with the output of the DC/DC converter being provided as input to a back-up battery and/or a DC/AC inverter. Current output from the DC/DC converter is taken by the battery as needed to compensate for loss of battery charge due to powering the load and/or self-discharging, thereby maintaining the battery fully charged. Power is undesirably lost by all of the current from the PV array going through the DC/DC converter prior to being supplied to the battery and/or the DC/AC inverter.
It is oftentimes desired to add a battery back-up to a grid-connected PV system initially installed without a battery back-up. The addition of a battery back-up to grid-connected PV systems initially installed without a battery back-up may be essentially precluded in some cases due to the extensive and costly modifications required. In other cases, grid-connected PV systems that are to be initially installed without a battery back-up must be specifically designed to permit the addition of a battery back-up at a later time, and this typically adds considerable cost and/or inefficiencies to the initial system.
A power system in which a load is powered by a battery when not being powered by an electric utility grid is disclosed in U.S. Pat. No. 6,353,304 B1 to Atcitty. The battery may be charged by a PV array which may also be used to power the load.
U.S. Patent Application Publication 2003/0072977 A1 to Speranza et al relates to a power system characterized by a renewable primary power source, such as PV, in electrical communication with a bus and a bridging power source including a battery. U.S. Pat. No. 3,600,599 to Wright discloses a power system having load and return bus lines.
PV power systems implemented in conjunction with commercial power sources or utility grids, but not having back-up power sources operable when power from the utility grids is unavailable, are represented by U.S. Pat. No. 5,493,155 to Okamoto et al, No. 5,522,944 to Elazari, No. 5,592,074 to Takehara, No. 5,929,538 to O'Sullivan et al and No. 6,448,489 B2 to Kimura et al and by U.S. Patent Application Publication 2003/0038713 A1 to Jansen. The Okamoto et al patent discloses a power system in which power can be supplied to a load in parallel from a PV array and a commercial power source. The commercial power source may be used to power a VVVF inverter to which a DC voltage output from the PV array is applied. The Elazari patent discloses a PV array electrically connected to a battery bank which is electrically connected to a DC/AC inverter. The inverter converts the DC current of the battery bank into 110/220VAC to power electrical appliances. When the voltage level of the battery falls below a predetermined level, the load is connected to the utility grid. The power system disclosed in the Takehara patent is characterized by a break device between the PV power system and the commercial power system. The O'Sullivan patent relates to PV power systems having a power processor capable of operating in an inverter mode, a battery charger mode and a parallel mode. The Kimura et al patent discloses a power system including a booster unit for boosting the DC voltage output of a solar cell string. U.S. Patent Application Publication 2003/0038713 to Jansen discloses a PV system having a bidirectional conversion circuit arranged between a DC voltage bus and the utility power grid.
U.S. Pat. No. 6,370,050 B1 to Peng et al and No. 6,344,985 B1 to Akerson disclose various DC/DC and AC/DC bidirectional power converters applicable to PV systems.
A need exists for grid-connected power systems in which a high voltage primary DC power source, such as PV, and a high voltage DC/AC inverter may be used in conjunction with a relatively low voltage back-up DC power source, such as a battery, for greater efficiencies and lower costs. There is also a need for grid-connected power systems in which direct current from a back-up power source is increased or decreased in voltage to be compatible with the voltage of a primary power source and a DC/AC inverter. Another need exists for grid-connected power systems in which direct current having a first voltage supplied from a high voltage primary power source to a high voltage DC/AC inverter is reduced to a lower second voltage in order to charge a low voltage DC back-up power source. A further need exists for grid-connected power systems in which direct current having a first voltage supplied from a primary power source to a DC/AC inverter is reduced to a lower second voltage when used to charge a back-up DC power source, and direct current from the back-up power source is increased in voltage from the second voltage to the first voltage prior to being supplied to the DC/AC inverter. An additional need exists for grid-connected power systems in which direct current output from a primary power source is supplied through a DC/DC converter only as needed to charge a back-up DC power source with the output from the DC/DC converter. There is also a need for grid-connected power systems in which alternating current from a utility grid or alternating current derived from the power system is converted to DC current of appropriate voltage to charge a back-up power source of the power system. Moreover, there is a need for grid-connected power systems in which alternating current from a utility grid is converted to direct current of lower voltage than the original voltage of the alternating current to charge a low voltage back-up power source, and the lower voltage direct current of the back-up power source is increased to a higher voltage prior to being supplied to a high-voltage DC/AC inverter. An additional need exists for grid-connected power systems utilizing alternating current from a utility grid to charge a back-up DC power source when the power system is electrically connected to the utility grid in a grid-connected mode and utilizing alternating current derived from a primary DC power source to charge the back-up power source when the power system is disconnected from the utility grid in a stand-alone mode. Still another need exists for back-up power sources provided as add-on modules for grid-connected power systems initially installed without the back-up modules, wherein the add-on capability is preserved without sacrificing efficiency and economy in the initial system.